The subject invention relates to a device and a process for the cooling of a stator and a rotor of an electric machine, where a gaseous cooling medium is circulated in a ventilation circuit by means of an externally driven fan.
In air-cooled generators, we differentiate basically between a so-called forward-flow and a reverse-flow cooling system, that is, on principle, according to the direction of flow of the cooling air through the generator. In forward-flow cooling, cold cooling air is extracted from heat exchangers with fans and pressed through the rotor, respectively, the air gap, and then the stator, from which hot cooling air exits and is recirculated via the heat exchanger. In the event of reverse-flow cooling, hot cooling air is extracted from the generator and fed to a heat exchanger, from where the cold cooling air flows to the rotor, respectively, the gap, through the stator.
Reverse-flow cooling systems for turbo-generators are known, for instance, from U.S. Pat. No. 5,633,543 A, U.S. Pat. No. 5,652,469 A, U.S. Pat. No. 6,346,753 B1 and EP 1 006 644 A2. JP 58 123 348 discloses a device and a process for cooling an electric machine with externally driven fans. However, the cooling device is directly mounted on the stator here, so that certain parts of the stator are in the warm air area. This can have negative effects on the width of the air gap between the stator and the rotor, because the edge areas of the stator are especially poorly cooled.
As described in the foregoing, the main characteristic of the reverse-flow cooling systems is suction-cooling of the generator, providing for the warm air being extracted from the generator using a fan and the cold air from the coolers being fed directly to the cooling channels of the generator stator. In most cases, the coolers are arranged on the pressure side of the fans, which is advantageous in that the temperature increase generated by the fans takes place before the cooler inlet and does not cause pre-heating of the generator. The flow in the stator is primarily directed from radially outside to inside and is normally extracted axially along the air gap, because turbo-generators normally have non-salient rotors. Given the high speed at which turbo-generator rotors run, a self-ventilation fan is arranged directly on, and driven by, the rotor shaft to produce the flow that is necessary for cooling.
In slow-running hydro-generators, which normally comprise salient poles, e.g. a bulb turbine generator, basically only externally-driven fans, i.e. fans that are not mounted on the rotor shaft—can be used, because the rotational speed of the rotor shaft would not suffice to produce the cooling air flow with a fan mounted on it. Hydro-generators with two rotating directions (e.g. a motor-generator) could constitute a second application for external ventilation.
Hydro-generators have so far only been operated with a forward-flow cooling system, as has been shown in JP 06 237 554 A2, for instance, where the cooler is mounted on the suction side and the generator on the pressure side of the fans and the flow in the stator is basically directed from radially inside to outside. When self-ventilation is used, a forward-flow-cooling system offers advantages in terms of the fan inflow and outflow. Inflow mostly takes place nearly without spin, which facilitates fan design. The spin-loaded outflow brings in its wake a pre-rotation of the rotor and therefore a reduction of the pressure loss at the rotor inlet (e.g. pole gap inlet).
If external fans are used instead of self-ventilation, these advantages are for the most part s eliminated. Several other disadvantages occur in connection with reverse flow: in hydro generators, especially salient-pole generators, major pressure loss is to be expected at the pole gap inlet. This pressure loss occurs radially outside in the air gap area, instead of radially centrally at the axial end sides as in the case of forward-flow. On account of the major pressure losses that are to be expected, a fan with higher power is needed, which in turn reduces the overall efficiency of the generator. In addition, depending on the arrangement of the fans, some pre-spin can persist at the fan inlet or the fan inlet flow can be more complicated than for the forward flow. It may also be so that the more complex flow at the fan inlet necessitates the use of guidance devices or rectifiers, which makes the cooling system design more complicated. For this reason, hydro-generators have so far not been operated with a reverse-flow cooling system but exclusively with forward flow.